The oxidative dehydrogenation of ethane to acetic acid and/or ethylene in the gas phase at high temperatures is known in the art. For example, U.S. Pat. No. 4,568,790 describes a process for oxidizing ethane to ethylene using an oxide catalyst. The calculated selectivity for ethylene at 50 percent conversion of ethane ranges from 63 to 76 percent. U.S. Pat. No. 4,524,236 describes a process for oxidizing ethane to ethylene using an oxide catalyst wherein the selectivity for ethylene at 51 percent conversion of ethane is as high as 80 percent. U.S. Pat. No. 5,162,578 describes a process for the selective preparation of acetic acid from ethane, ethylene or mixtures thereof with oxygen that results in an acetic selectivity of 34 percent and an ethylene selectivity of 62 percent with an ethane conversion of 4 percent. A further process for the preparation of a product comprising ethylene and/or acetic acid is described in European Patent No. EP 0 407 091 B1. According to this process, ethane and/or ethylene and a gas containing molecular oxygen is brought into contact at elevated temperature with a mixed metal oxide catalyst, resulting in a maximum selectivity for acetic acid of 78 percent at 14.3 percent ethane conversion. The highest selectivity for ethylene was 70 percent at 15 percent ethane conversion.
Vinyl acetate is generally prepared commercially by contacting acetic acid and ethylene with molecular oxygen in the presence of a catalyst active for the production of vinyl acetate. Integrated processes for producing vinyl acetate are also known in the art. For example, U.S. Pat. No. 6,852,877, which is incorporated by reference herein in its entirety, discloses a process for the production of vinyl acetate comprising (1) reacting ethane with oxygen in the presence of a catalyst to produce acetic acid (ethane oxidation), (2) reacting ethane with oxygen in the presence of a catalyst to produce ethylene (ethane oxidative dehydrogenation); (3) reacting the ethylene and acetic acid produced above with oxygen in the presence of a catalyst to produce a vinyl acetate product stream; and (4) separating the vinyl acetate from the product stream from step (3).
Furthermore, commonly owned U.S. Pat. No. 6,790,983, which is incorporated by reference herein in its entirety, discloses a process for the production of vinyl acetate comprising (1) reacting ethane with oxygen in the presence of a catalyst to produce acetic acid and ethylene (ethane oxidation), (2) reacting the ethylene and acetic acid produced above with oxygen in the presence of a catalyst to produce a vinyl acetate product stream; and (4) separating the vinyl acetate from the product stream from step (2).
FIG. 1 shows a common prior art ethylene/acetic acid production process. In this basic system, an ethane containing stream (1) is fed along with an oxygen containing gas (2) into an ethane oxidation reactor (3). This reactor can be either a fluidized bed or fixed-bed reactor. Inside the reactor (3), ethane is oxidized into acetic acid, ethylene, and various carbon oxides (COx). The gaseous reactor effluent (4) that contains these three primary components is fed into a recycle gas scrubber (5), which produces a top stream containing ethylene, ethane, and COx, and a bottom stream (6) which contains acetic acid, water, and heavy ends by-products. The bottom stream (6) is then purified as known in the art to provide purified acetic acid for sale or for use in a downstream vinyl acetate process (not shown). The top stream (7) from the recycle gas scrubber is routed to a processing step (8) that removes the COx from the top stream. The purified stream is then fed to an ethane/ethylene separator, a separation that is often costly and difficult to achieve. The ethane stream (10) is recycled to the oxidation reactor (3) for further conversion into acetic acid, while the purified ethylene stream (11) is sent to a downstream unit, such as a vinyl acetate unit, or stored for future sale.
A common problem in the production of acetic acid/ethylene is purifying the ethylene stream for commercial production or for feedstock to a vinyl acetate plant. When ethane and/or ethylene is oxidized to produce acetic acid and/or ethylene, the product stream contains both ethane and ethylene. The separation of these two components is very challenging. It would therefore be desirable to develop a process where ethylene can be easily separated from ethane so as to produce ethylene for sale or for use in a downstream process, such as a vinyl acetate plant.